Radio failsafe fix

Hi folks

** PROPOSED FIX **

I've proposed a fix and here are the changes:

https://github.com/diydrones/ardupilot/pull/357/files

I think it solves the problem but I'm unwilling to try it until a few people have cast their eye over it (given it involves flying a distance away). 

I've rearranged the read mode switch function so that it doesn't update the old mode in failsafe mode - otherwise it'll detect a change after failsafe off. Also set FS_COUNTER to times 2 on failsafe off event, so 6 = ~300ms.

Best
Gareth